One of the principle sources of parasitic losses, complications and bulk in an internal combustion engine has to do with the waste heat generated by the internal combustion engine process. Attempts have been made to manage heat flux from the material surrounding combustion chambers by paying careful attention to flow passages, coolant flow rates and temperatures through such passages. Typically, internal combustion engines are liquid cooled so as to maximize the heat flux to the cooling system, particularly in the region closely adjacent the combustion chamber. When cooling systems operate under off design conditions because of duty cycle or component malfunction, it can lead to a condition of uncontrolled boiling in the coolant passages for the engine. This condition causes complete loss of liquid to metal contact and drastically reduces the heat flux carried away by the cooling system. When this is left uncontrolled, the pressure relief for the system, usually a radiator cap, is opened to release pressure and allow even greater generation of steam. This, in turn, has a potentially catastrophic affect on the temperature of the internal metal parts of the engine.
There is, however, a condition between normal liquid flow conditions and uncontrolled boiling that provides an optimum heat flux from the parts to be cooled by the liquid cooling system. This is known as nucleate boiling in which bubbles are generated on a microscopic scale. This allows significant increases in heat flux, but this condition, at best, is a momentary transition between sub-boiling conditions and uncontrolled or macro-boiling.
It has been proposed in copending application Ser. No. 12/136,197, filed on Jun. 10, 2008, of common assignment with the present invention, to provide active control of the pressure within the cooling system of an internal combustion engine. This is done with a sensor and a device to increase or decrease, on a rapid basis, the system pressure so as to maintain nucleate boiling and the optimum heat flux. One example of such a device is a second pump activated to increase pressure. Such a system, while effective in maintaining conditions of nucleate boiling, has additional complications and complexity which increases the cost of such a system for an internal combustion system.
What is needed in the art therefore is a cost effective system for controlling pressure to produce nucleate boiling.